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Potential risks of Stratospheric Aerosol Injection (SAI): ozone depletion

Summary​

Stratospheric Aerosol Injection (SAI) poses significant risks to Earth's ozone layer, which protects life by absorbing harmful UV radiation. The primary mechanisms involve sulfate aerosols enabling ozone-depleting chemical reactions and potential increases in stratospheric water vapor that breaks down ozone (O₃). While volcanic eruptions like Mt. Pinatubo demonstrated sulfate aerosols' cooling effects, they also caused measurable ozone loss. Current research suggests controlled SAI might reduce this impact through optimized deployment, but uncertainties remain about water vapor interactions and net UV effects. Key studies show conflicting possibilities, with some indicating potential UV reduction depending on aerosol placement altitude. The canceled SCoPEx experiment aimed to better understand these atmospheric interactions.

Potentially the largest concern regarding SAI is its effect on the ozone layer 1. The ozone layer is a region of the Earth's stratosphere that contains a high concentration of ozone (O₃) molecules. It is located approximately 10 to 30 kilometers (6 to 19 miles) above the Earth's surface and plays a crucial role in protecting life on our planet. The ozone layer absorbs and blocks most of the Sun's harmful ultraviolet (UV) radiation, particularly UV-B and UV-C rays, which can cause skin cancer, cataracts, and other health issues in humans, as well as damage to ecosystems, marine life, and crops. By filtering out these dangerous rays, the ozone layer acts as a protective shield, maintaining the health and stability of the Earth's environment and supporting life as we know it 2.

SAI can deplete the ozone layer through two mechanisms: the facilitation of ozone depletion by sulfate aerosols, and the indirect depletion of ozone via increases in stratospheric water vapor 3.

Sulfate aerosols can facilitate ozone depletion by providing surfaces for reactions.​

Sulfate aerosols are often presented as the most suitable for SAI due to their well-understood atmospheric dynamics and cost-effectiveness 1. Sulfate aerosols are also produced naturally from volcanic eruptions, which has acted as a proof of concept through examples like Mt. Pinatubo in 1991, which injected 20 million tons of sulfur dioxide (SO₂) into the atmosphere, reducing global temperatures by approximately 0.9°F for over a year.

Sulfate aerosols, which in most cases refer to sulfuric acid (H₂SO₄), do not react chemically with ozone molecules. Instead, the sulfuric acid molecules provide a surface for reactive particles such as chlorine (Cl) or hydrochloric acid (HCl) to react with ozone, as the chlorine-containing molecules and ozone particles can attach to the sulfate aerosol where they have sufficient time to react with each other 4.

The eruption of Mt. Pinatubo resulted in the loss of around 2.5% of global ozone, but it is estimated that an intentional sulfate injection in the stratosphere would require a quantity of sulfur five times lower to have the same cooling effect as Mt. Pinatubo, resulting in less ozone destruction 1.

Sulfate aerosols can increase the amount of stratospheric water vapor, which can deplete ozone.​

The presence of water vapor in the stratosphere can lead to the depletion of ozone through a series of chemical reactions. Water vapor is first broken down by UV radiation to form hydroxyl radicals (OH), a molecule that breaks down ozone. It is proposed that SAI might increase the levels of water vapor in the stratosphere by locally increasing the temperature of the stratosphere, allowing the air to hold more water. This is caused by the local light scattering of the aerosol particles, as well as the absorption of radiation that results in localized heating. A secondary effect of local increases in stratospheric temperatures is the higher levels of convection (the forced movement and mixing of air) which can be caused by temperature gradients along the length of the atmosphere. This increased convection can carry water vapor from the troposphere up into the stratosphere where there are high concentrations of ozone. The influences of sulfate aerosols on localized temperatures in the stratosphere are not well understood, and there have been arguments made that aerosols might actually reduce stratospheric water vapor 3. Understanding the connection between aerosols and water vapor was one of the main goals of the Stratospheric Controlled Perturbation Experiment (SCoPEx) experiment, which was canceled in March of 2024.

The physical mechanisms between using sulfate aerosols for SAI and ozone depletion are well understood, but the extent to which this will occur is still uncertain. The main concern regarding ozone depletion is that there will be a damaging increase in UV radiation. However, some studies have suggested that depending on the altitude of the placement of sulfate aerosols relative to the ozone layer, there may be a net reduction in UV radiation 5.

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Footnotes​

  1. Crutzen PJ. (2006). Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma? Climatic Change, 77(3), 211-220. https://doi.org/10.1007/s10584-006-9101-y ↩ ↩2 ↩3

  2. Hollósy, F. (2002). Effects of ultraviolet radiation on plant cells. Micron, 33(2), 179-197. https://doi.org/10.1016/S0968-4328(01)00011-7 ↩

  3. John Dykema, David Keith, James G. Anderson, & Debra Weisenstein. (2014). Stratospheric controlled perturbation experiment (SCoPEx): a small-scale experiment to improve understanding of the risks of solar geoengineering. Philosophical Transactions of the Royal Society A, 372. https://doi.org/10.1098/rsta.2014.0059 ↩ ↩2

  4. Solomon, S. (1999). Stratospheric ozone depletion: A review of concepts and history. Reviews of Geophysics, 37(3), 275–316. https://doi.org/10.1029/1999RG900008 ↩

  5. Madronich S, Tilmes S, Kravitz B, MacMartin DG, & Richter JH. (2018). Response of Surface Ultraviolet and Visible Radiation to Stratospheric SO2 Injections. Atmosphere, 9(11), 432. https://doi.org/10.3390/atmos9110432 ↩